Methylmercury and Omega-3 Fatty
Acids: Co-occurrence of Dietary Sources
and the Role of Fish and Shellfish
Kathryn R. Mahaffey1 and Elsie M. Sunderland2
1George Washington University, School of
Public Health, Washington, D.C.
2Harvard School of Engineering and Applied
Sciences, Cambridge MA
• Amounts of omega-3s recommended for
• Dietary sources of omega-3 fatty acids
• How to optimize the intake of omega-3 fatty
acids and control exposure to
Why are the essential fatty acids
• Growth • Maintain membrane
• Neurological and visual fluidity and confirmation
development. • Cell signaling pathways.
• Immune function • Synthesis of
• Gene expression physiologically important
• Structural lipids of the chemicals, e.g.
nervous system prostaglandins.
• Platelet aggregation
• Vessel wall constriction
Omega-3 Fatty Acids
• Fatty acids contain carbon, oxygen, and hydrogen in repeating groups of –
(CH2 )n – with a methyl (CH3) group on one end and a carboxyl (-COOH)
group on the other end of the chain.
• Fatty acids with a double bond between the carbon atoms (-C=C-) have to
be supplied in the diet because humans cannot synthesize them.
Alpha-linolenic (18:3 omega 3)
Used to synthesize:
Eicosapentaenoic or EPA (20:5 omega 3),
Docospentaenoic (22:5 omega 3) or DPA and
Docosahexaenoic (22:6 omega 3) or DHA
Omega-3 Fatty Acids
• α-linolenic acid comes from dietary oils –
predominantly soybean oil and flax seed oil.
• EPA and DHA
synthesized from the precursor α-linolenic
supplied preformed from the diet.
Human Capacity to Synthesize EPA
and DHA from ά Linolenic Acid
• If alpha linolenic is in the diet humans can make some
EPA and DHA. Adult males seem to form less than 10%
of the amount that is needed.
• Women, especially during pregnancy, are able to form
EPA and DHA at a higher rate because of the effects of
• Fetus depends on transfer of EPA and DHA from the
mother - Both are important to optimal neurological status
Dietary Sources of Omega-3s
• Algae – basic source.
• Fish, shellfish, and marine mammals are the usual sources, but depend
on algae for basic synthesis.
• Other animals (chickens, beef) can be a source of omega-3 fatty acids
if these animals are grown “free range” or are able to graze in the open.
• Special diets fed to chickens can produce eggs containing more than
600 mg of omega-3 fatty acids per each 100 gram egg.
• Food supplements such as fish oil or supplements based on algae.
• Biotechnologically produced omega-3 fatty acids; e.g. from
Omega-3 Fatty Acids in Eggs
• Hens fed a special diet containing flaxseed, canola oil, sea
algae or other omega-3 rich products.
• One egg provides 100 mg to 660 mg (Christopher Eggs)
compared with ~ 40 mg/egg in traditional eggs. An egg
weighs ~ 50 grams. 200 mg to 1320 mg/100 gram portion.
• Yearly per capita egg consumption of eggs in 2001 was
252 eggs (Pickering, 2003) in the United States.
• “Designer” eggs are ~ 5% of the egg market (Hander,
2001) in the United States. Omega-3 eggs are likely 2% or
3% of the total egg market, but firm figures are not readily
How much fish do you need?
• Mackerel provides 1790 mg combined EPA and DHA/100
gram serving. Takes ~ 75 grams of mackerel to provide
1350 milligrams of combined EPA and DHA.
• Cod contains 250 mg combined EPA and DHA/100 gm
serving. Takes ~560 grams of cod to provide 1350
milligrams combined EPA and DHA.
The Balance: Omega-3s and
Methylmercury in Seafood
• Recommendations of 1600 milligrams of omega-3 fatty
acids for men and 1100 milligrams of omega-3 fatty acids
for women (US NAS/NRC Dietary Recommendations) =
average 1350 milligrams/day.
• Reference dose for CH3Hg based on fish consuming
• Little association between CH3Hg in fish and DHA in fish.
• Can have nutritional benefit from fish and still have low
Comparison with DHA Enriched Eggs
• Special eggs can contain ~ 650 mg EPA and DHA per egg.
• One egg weighs approximately 100 grams.
• Two eggs can supply the recommended ~ 1300 mg omega-
3 fatty acids.
• Cost in US $ is about $.30/egg or ~ $0.60 for two eggs.
• Cost of fish is species and location dependent.
• How Much of a Particular Fish Species
is Needed to Get the Recommended
Amounts of EPA + DHA (1350
• If we know how much Hg is in that
fish species, then
• What is the Resulting Hg Exposure?
Concentrations of Mercury and Omega-3 Fatty Acids
(EPA + DHA) in Selected Fish and Shellfish Species
Pollock Breaded fish products Perch
Cod Crayfish Oysters
Mackerel M ussels Scallops Haddock
Omega-3 fatty acids, g/100g fish
T una (canned) Swordfish
T rout Bass T una (fresh/frozen)
Catfish King Mackerel
Clams Pike T ilefish
0 0.25 0.5 0.75 1 1.25 1.5 1.75
Mercury, ug/g fish
Source: Mahaffey et al., 2008
Omega-3 Content of Fish & Shellfish Vary Widely
- Virtually All Contain Methylmercury
Fish EPA + Hg Gms of Fish Hg intake μg Hg per
DHA for 1350 kg bw for a
Species Mg/ 100
of Fish 1350 mg mg EPA + 70 kg adult
gms DHA in
EPA + DHA
of Fish this fish
Mackerel 1790 0.087 75 6.5 0.09*
Salmon 1590 0.035 85 3.0 0.04*
Swordfish 580 0.950 230 220 3.2
Cod 240 0.121 560 68 0.97
Correlation between Total Fish Intake and Mercury Intake, both normed
to body weight (Pearson Correlation R=0.68, p<0.001, n=509).
(Mahaffey et al., Env Res 2008)
Women 16-49 years who reported consuming fish in 24-hr dietary recall
(minus 1 outlier)
Mercury intake (μg/g bw)
y = 0.1045x + 0.0374 Shrimp
R2 = 0.4596
0 1 2 3 4 5 6 7 8 9 10
Fish consumption (g/kg bw) 15
Omega-3 Fatty Acid (EPA + DHA) and Mercury Intake from Fish
(Pearson Correlation R=0.66, p<0.001, n=3,614).
(Mahaffey et al., Env Res 2008)
Women 16-49 years 24-hr recall data (minus 1 outlier)
Omega-3 fatty acid intake
y = 0.028x + 0.0003 Breaded Fish
R2 = 0.4387 Products
0 0.3 0.6 0.9 1.2 1.5
Mercury intake (μg/kg bw) 16
Omega-3 Fatty Acid (EPA + DHA) Intake from All Foods and Mercury
Intake from Fish Consumption (Pearson Correlation R=0.52, p<.0001,
n=3,614) (Mahaffey et al., Env Res 2008)
Women 16-49 years, 24-hr recall data (minus 1 outlier)
Omega-3 fatty acid intake
y = 0.0235x + 0.0008 Salmon
R2 = 0.2675
0 0.3 0.6 0.9 1.2 1.5 17
• Nutritionists recommended a 4-fold increase in fish
consumption in order to attain the proposed recommended
combined EPA and DHA intake of 650 mg/day (Kris-
Etherton et al., 2000).
• If levels of 1600 for men and 1100 mg/day are used this
would be approximately an 8-fold increase in fish intake.
• This would result in an increase in exposure to
methylmercury and other contaminants.
The Balance for Fish Consumption,
Methylmercury Exposure, and
Persistent Organic Pollutant (POP) Exposures
• Good news: Can maintain low mercury exposures
and still have adequate DHA intake because
methylmercury binds to proteins in tissues and
DHA concentrates in lipoprotein membranes.
• Bad news: Likely to be very difficult to have
adequate DHA intake from fish and low exposures
to POPs because both are in tissue lipids.
If fish alone were relied upon to increase the intake of
omega-3 fatty acids the following problems would occur:
• Potential for severe depletion of marine fish.
• Need to rely more on other sources
• If there were an 8-fold increase in fish intake,
there would be increased exposure to
contaminants. How severe the increase is depends
on pollution and on the fish species.
• Pollution control – the most important solution. But may
be a very long term solution.
• Use of algae.
• Use of biotechnology.
• Use of diet modification to increase the oemga-3 fatty
acids in other non-fish foods.
• Use of food enrichment with EPA and DHA. In US
approximately 100 food products have DHA added to
Important Data Needs
• More quantitative data on human ability to
synthesize EPA and DHA from ά-linolenic acid.
• More information on which species of fish are
consumed in individual countries.
• More data on the EPA and DHA concentrations of
specific fish species.